As an exhaust gas purifying catalyst to be arranged in an exhaust system of a diesel engine or the like, an oxidation catalyst (Diesel Oxidation Catalyst: DOC) for oxidizing hydrocarbons (HC), carbon monoxide (CO), and nitrogen monoxide (NO) contained in exhaust gas is known. In addition, a diesel particulate filter (DPF) for collecting particulate matter (PM) contained in the exhaust gas and other filtering devices are also known.
If a capability of the DOC to oxidize HC is degraded, a portion of HC which is supplied to the DOC by an exhaust pipe injection (in-pipe injection) or the like at the time of a forced regeneration of the DPF will experience a slip to the DPF on the downstream side without being oxidized by the DOC. Because the DPF also has a capability to oxidize HC, the portion of HC which has experienced a slip through the DOC may be oxidized and purified by the DPF. However, if HC oxidation performance of the DPF is also degraded, unburned HC which has experienced a slip through the DOC may pass through the DPF and released to the atmosphere. This may worsen undesired emissions. Thus, there is a demand to diagnose the capabilities of the DOC and the DPF to oxidize HC when the DOC and the DPF are on board (see, forexample, Patent Literature Document 1).